The present invention relates to an improved method for crystallizing stretch blow molded thermoplastic containers, especially polyethylene terephthalate (PET), by sequential heat treating to induce crystallization thereof.
According to current practice, of which U.S. Pat. No. 4,836,971 is an example, a partially crystallized container is made from an amorphous PET preform by crystallizing its neck, heating just its body to the orientation temperature of PET, blow molding an intermediate container in a cooled mold with a larger volume than that of the final container, heating the intermediate container to 180.degree. to 220.degree. C. for 1 to 15 minutes, and then re-shaping by blow-molding a second time to the final container dimensions. Such heating the intermediate container results in considerable deformation and shrinkage of the container, which necessitates the final molding operation, due to stress relief and, because said heating causes the formation of crystalline regions which have a greater density than the amorphous phase. Depending upon the amount and nature of the shrinkage, the re-shaping step is difficult to carry out and to control.
As is well known, it is highly desirable to crystallize molecularly oriented containers in order to improve their properties: crystallinity improves resistance to gas-permeation and strength and reduces gas solubility. In combination, these properties can provide a superior container to retain or exclude fluids, to avoid dissolving them in the container walls and to reinforce the structure. However, for these improvements to be worthwhile, high degrees of crystallinity must be obtained, e.g. 45% in the case of PET, which can only be accomplished at high temperatures for the process to be economical.
However, it is important to obtain the desirable properties of the crystalline phase without excessive shrinkage during the heat treating step. At such temperatures, attained in a short time, the deformation and shrinkage becomes irregular and excessive, and difficult to offset in the course of re-shaping.
It is known that deformation and shrinkage, as above, occur upon heating the molecularly oriented container, due to the relief of residual stresses, and increasingly with increasing temperature, due to the density of the crystalline phase. The treatment that results in such stress relief and relatively low levels of crystallinity is termed "heat-setting" and it is used widely to produce bottles that may be filled with liquid at elevated temperatures without
Therefore, it is a principal object of the present invention to provide an efficient process for heat treating thermoplastic containers, especially of polyethylene PG,4 terephthalate (PET), wherein stretch blow molded, molecularly oriented thermoplastic containers are highly crystallized in an efficient and effective manner by heat setting them before final crystallizing, thereby controlling shrinkage so as to facilitate the final shaping thereof.
It is a further object of the present invention to provide a process as aforesaid which is effective, expeditious and convenient to use on a commercial scale.
It is a still further object of the present invention to provide a process as aforesaid which obtains improved containers having greatly improved properties.
Further objects and advantages of the present invention will appear hereinbelow.